This invention relates to a gas-flow laser device, and more particularly to a coaxial type gas-flow laser device which exhibits a stable discharge.
In a high-power CO.sub.2 gas laser device, forced convectional cooling is adopted as a gas cooling method. The gas is circulated by a blower, and its temperature having risen in a discharge portion is lowered by a heat exchanger.
The gas-flow laser device has three axes, i.e., a laser beam axis, a gas flow axis and an electric discharge axis. The laser device is divided into the following three modes by the combination of the directions of the three axes; (1) a coaxial type gas flow laser device in which three axes are coincident, (2) a cross beam type gas flow laser device in which a gas flow axis and an electric discharge axis are coincident and a laser beam axis intersects orthogonally with the other two axes, (3) a 3-axes orthogonal-intersection type gas flow laser device in which the three axes intersect orthogonally to one another.
In a coaxial type gas laser device, the pressure loss of a gas flow passage is, for the most part, attributed to the electric discharge. In case of considering devices of equal maximum laser outputs, the pressure loss in the coaxial type laser device is approximately 10 times greater than in a 3-axes orthogonal-intersection type gas laser device wherein the three axes intersect orthogonally to one another or a cross beam type laser device wherein only the beam axis intersects orthogonally to the other two axes. In the coaxial laser device, accordingly, there has heretofore been employed a Roots blower which is a displacement blower capable of providing a high discharge pressure even under a low gas pressure of 10-50 Torr. For example, a coaxial gas flow laser device provided with a Roots blower is shown in the publication "THE WELDING INSTITUTE RESEARCH BULLETIN, pages 29-33, Feb. 1976". It is unavoidable that the gas flow becomes a pulsating one which has lowered flow rate parts in a proportion of two times per revolution of a rotor shaft. The inventors' investigation on the transition phenomenon of the glow discharge to the arc discharge has revealed that, in the case where the Roots blower is employed, the flow rate of the gas fluctuates considerably in correspondence with the rotation of the blower, the arc transition being initiated at a part at which the flow rate is the smallest.
It is known that the laser output is proportional to the injection input in the glow discharge. In addition, the upper limit of the injection input in the glow discharge is proportional to the quantity of gas flow. As the flow rate is smaller, the transition to arc discharge occurs at a lower discharge input. Since the arc discharge is scarcely concerned in the excitation of laser gas molecules or atoms, the glow discharge becomes unstable and the function of the laser device is not effected. So, in order to obviate the instability of glow discharge, a restriction like an orifice or a nozzle is provided at the upper stream of the discharge tube and the shockwaves are generated. However, since the restriction increases the pressure loss at the restriction part, the ratio of an inlet pressure P.sub.i to an outlet pressure P.sub.o of the blower must be increased for obtaining more than 1 kW high power laser. Consequently, there is a disadvantage that the driving force of the blower should be increased and the whole size of the laser device should be large.
As a method of suppressing an increase in a driving force as is brought about by the increase of the blower pressure ratio owing to the adoption of the restriction, it is considered to make the period of the pulsations extremely short as compared with a period of time in which the laser gas passes through the discharge portion. More specifically, in attaining a high power of 1-2 kW/min. With a coaxial type laser device circulating a gas at high speed, the period of time in which laser gas molecules pass through a discharge portion is recommended to be at most 10 millisecond, and it is usually selected to be 2-5 millisecond. When the period of the pulsations is made sufficiently short as compared with the specified time, that is, when the rotational frequency of a rotor shaft is raised, the influence of the pulsating flow can be lessened. However, the rotor shafts are in mechanical mesh in the Roots blower. Even in a small-sized Roots blower, therefore, a realizable rotational frequency is limited to 5,000 r.p.m. Even in this case, the period of the pulsating flow becomes 6 millisecond. It has been difficult to attain a good glow discharge, i.e., a stable laser output without adopting any nozzle.
On the other hand, in a cross beam type or a 3-axes orthogonal-intersection type laser device, the gas flows in a state of plane instead of axial flow as in a coaxial type laser device. Since the pressure loss is small, an axial-flow blower or a centrifugal blower may be employed. U.S. Pat. No. 3,921,098 shows a cross beam type laser device using an axial-flow blower. Japanese utility model publication No. 52-43981 (filed on 1977) shows one using a centrifugal blower. In these prior arts, when the gas pressure increases, the discharge becomes unstable and the reliability of the laser decreases.